My (23m) Girlfriend (24f) is pissed that she isnt invited to my best friends wedding. What should i do?

My girlfriend isnt invited to my best Friends wedding.

So im the best man at my best friends wedding, but i cant bring my girlfriend. As far as i know No one is allowed to bring a +1 because they cant afford the number of guests to double. The problem with this is, that my girl is very pissed about this. I tried to talk to my friend about it and offered to pay for her if it is because of the money. He still said no. My girlfriend is of the opinion that she isnt allowed to come, because the bride doesnt want her to steal the show, because she is very attractive. I cant deny or confirm this but i dont know of any fights or arguments between them that could be a reason to not invite her.

If i dont manage to talk my friend into also inviting her, she wants me to kind of end the friendship, and if we should get married, she also wouldnt invite them.

I dont know what to do. I cant seem to convince my friend to invite her, but i also dont want to lose my best friend.

Update: I may need to give more info. No one is allowed to bring a plus one, except one Person, that is the second best man. He brings his partner, who he is engaged to though. My GF knows of that and thinks there are Personal reasons that i cant bring her.